If
the scriptures were preserved in memory for so long they must be very unreliable.
Much of the Buddha's teachings could have been lost or changed?

The preservation
of the scriptures was a joint effort by the community of monks and nuns.
They would meet together at regular intervals and chant parts or all of
the Tipitaka. This made it virtually impossible for anything to be added
or changed. Think of it like this. If a group of a hundred people know a
song by heart and while they are all singing it one gets a verse wrong or
tries to insert a new verse, what will happen? The sheer number of those
who know the song correctly will prevent the odd one making any changes.
It is also important to remember that in those days there were no T.V.'s,
newspapers or advertising to distract and clutter the mind which together
with the fact that monks and nuns meditated, meant that they had extremely
good memories. Even today, long after books have come into use, there are
still monks who can recite the whole Tipitaka by heart. Mengong Sayadaw
of Burma is able to do this and he is mentioned in the Guinness Book
of Records as having the world's best memory.